Inhibitory effects and biotransformation potential of ciprofloxacin under anoxic/anaerobic conditions

The inhibitory effects and biotransformation potential of the fluoroquinolone antibiotic ciprofloxacin (CIP) under anoxic (i.e., nitrate reducing) and anaerobic (i.e., sulfate reducing and methanogenic) conditions were investigated. Fermentation and sulfate reduction was inhibited in 10–80 mg/L CIP-amended sulfate-reducing cultures but recovered with prolonged incubation. Methanogenesis in the mixed culture was significantly inhibited at 80–100 mg CIP/L. No significant decrease of CIP concentration was observed under both sulfate-reducing and methanogenic conditions. However, a low degree of CIP biotransformation was observed in a fed-batch denitrifying culture after a lag time even though the microbial, denitrifying activity was gradually inhibited at 24–40 mg CIP/L. Furthermore, the degradation of CIP was accelerated with a CIP reamendment of the denitrifying culture. Two CIP biotransformation products in the denitrifying culture were detected and their proposed chemical structures suggest that the antibiotic quinolone moiety of CIP was intact.